[Bug]: GetExtendedAgentCard request do not support the optional tenant parameter

[jmesnil]

What happened?

The client must provide an optional tenant field when calling the method to get the extended agent card:

https://github.com/a2aproject/A2A/blob/92e6fb712ebeddd732a0a124b95d5ff8db4e0d41/specification/grpc/a2a.proto#L849

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